The invention relates to a process for measurement of the liquid absorption of powdery solids. The invention aims at reproducibly determining the maximum possible amount of liquid that powdery solids can absorb up to saturation.
Powdery solids, such as, for example, mineral fillers for rubber processing, carbon blacks and pigments, have the property of being able to absorb liquids without losing their powder form. The amount of liquid that the powdery solids can absorb at most, i.e., until saturation occurs, is especially important. A powdery solid--hereinafter called powder--is a mineral filler, a pigment or a carbon black, whose bulk density can vary in a broad range, down to extremely low values of the bulk density.
The liquid absorption--hereafter abbreviated LA--is used to determine the surface properties of the powder and gives indications of the powder structure and the processing properties of the powder. Examples of this are the "DBP number" (Dibutylphthalate number) in carbon blacks and light-colored fillers as well as the "oil number" in pigments and fillers.
According to DIN 53 601 (Dec. 78) and ASTM D 2414-82 dibutylphthalate (DBP) absorption of carbon blacks is determined with the Cabot absorptometer. For this purpose, the amount of DBP is measured which was dripped at room temperature onto the carbon black stirred in the maximum absorptometer until reaching a preset torque, which is clearly below the torque. The empty volume of the carbon black can be estimated and indications of its processing properties can be derived from the DBP absorption. In DIN 53 601 the oil feeding is automatically cut off at a torque of about 70% of the maximum occurring torque. Further, two calibrating carbon blacks are used in this process.
The kneading chamber of the Cabot Absorptomer has a volume of about 50 cm.sup.3, the rotor pair runs with a rotation ratio of 125 to 250 per minute.
Generally 20 g at a bulk density of 0.5 g/cm.sup.3 is recommended as carbon black amount. The degree of fullness then is 80%. In any case, the kneading chamber must be adequately filled, since false measurements result in insufficiently filled kneading chambers. The DBP runs with constant speed of 4 ml per minute (but pulsing) on the stirred carbon black.
It is further known, according to the oil number according to DIN ISO 787 (Feb. 1983) to determine the amount of refined linseed oil, which is absorbed by a pigment or filler sample under established conditions. In this case a sample amount of pigment or filler is put on a glass plate and some drops of refined linseed oil are dropped on it at room temperature. The oil is rubbed into the sample amount with a knife spatula. Dropping of oil and rubbing it into the sample amount are repeated several times, until a soft paste results, i.e., until the powder has lost its powder form. Recognition of the end point of the oil addition depends on the tester. The oil numbers determined according to this process therefore are preferably considered as relative values of a testing position (between sample and comparison sample).
In both methods the liquid (DBP or refined linseed oil) is added to the stirred or rubbed powder at room temperature, as pulsing sharp jet in the Cabot Absorptometer or as drops in the oil number method. Thus, the powder particles are locally overloaded with liquid, as a result of which the measurement results are adversely affected.